1. Field of the Invention
The present invention relates to a four-cylinder engine in which the explosion intervals of four cylinders are 270°, 180°, 90°, and 180° as crank angles, and a method of operating the four-cylinder engine.
2. Description of the Related Art
As a crankshaft of a four-cylinder engine mounted in a vehicle, a one-plane-type crankshaft (180° crankshaft) in which four crank pins are positioned on one virtual plane and a two-plane-type crankshaft are described in Japanese Patent No. 4533846. Crank pins of the two-plane-type crankshaft are arranged two by two on two virtual planes perpendicular to each other when viewed in the axial direction of the crankshaft.
It is known that a vehicle equipped with a four-cylinder engine using the two-plane-type crankshaft of this kind or a V-type four-cylinder engine including the one-plane-type crankshaft gives an occupant the feeling that an accelerator operation and the behavior of the vehicle body match. This is so probably because this crank layout removes most of the inertial torque of the engine. The explosion intervals of the four-cylinder engine of this kind are often 270°, 180°, 90°, and 180° as crank angles.
The combustion pressure and generated torque of the four-cylinder engine as described above change as shown in FIG. 28. In FIG. 28, the abscissa indicates the crank angle, and the ordinate indicates the combustion pressure and generated torque. Also, FIG. 28 shows an example of a case in which the ignition order is first cylinder→third cylinder→second cylinder→fourth cylinder. In this case, when explosion occurs in the first cylinder at a crank angle indicated by A in FIG. 28, explosion occurs in the third cylinder at crank angle B advanced 270° from crank angle A. Then, the crankshaft further rotates 180°, and explosion occurs in the second cylinder at crank angle C. After that, the crankshaft rotates 90°, and explosion occurs in the fourth cylinder at crank angle D. When the crankshaft further rotates 180° and reaches crank angle E after that, explosion occurs in the first cylinder again. On the other hand, torque generated on the output shaft of this four-cylinder engine rises at the same intervals as the explosion intervals. That is, torque is generated at unequal intervals corresponding to the unequal-interval explosions.
A four-cylinder engine in which the explosion intervals are 270°, 180°, 90°, and 180° as crank angles poses a problem when the engine speed is low. This problem is that an occupant of a vehicle equipped with the engine of this kind feels a sense of discomfort. The main cause of this sense of discomfort is that the explosion intervals of the four cylinders are unequal so the driving energy per unit time becomes non-uniform. The driving energy herein mentioned is energy that rotates the output shaft of the engine (and the driving wheels of the vehicle). The magnitude of torque to be applied to the output shaft changes in proportion to the magnitude of the driving energy.
The above-described torque rises twice within a short time when explosions occur in two cylinders having an explosion internal of 90° as a crank angle. The torque rises twice within a short time as described above when, for example, explosion occurs in the second cylinder at crank angle C and occurs in the fourth cylinder at crank angle D in the engine shown in FIG. 28. That is, since the torque is generated in succession within a short time as described above, the driving energy rises in a portion of one cycle, so the driving energy per unit time becomes non-uniform as described above.
An occupant of a vehicle equipped with this four-cylinder engine sometimes experiences the rises of torque occurring in succession within a short time as one torque rise when the engine speed is low. The occupant often feels that the magnitude of this torque experienced as one torque rise is larger than that of torque which rises in each of the two other explosions. This is so because the torque rises twice within a short time, so the occupant feels that the torque rise time is long. That is, as indicated by the alternate long and two short dashed line in FIG. 28, the occupant feels that the torque is generated once although it is actually generated twice, and, in addition to that, misunderstands that the generated torque is larger than the torque generated when explosion occurs in another cylinder (the first or third cylinder).
Consequently, the occupant experiences an irregular torque change when the engine speed is low, and feels a sense of discomfort.